1. Field of the Invention
The present invention relates to a method for manufacturing a composite piezoelectric substrate including a piezoelectric thin film and a piezoelectric device including the composite piezoelectric substrate. More particularly, the present invention relates to a method for manufacturing a composite piezoelectric substrate in which a piezoelectric thin film is subjected to heat treatment and a piezoelectric device including the composite piezoelectric substrate.
2. Description of the Related Art
Various piezoelectric devices that utilize piezoelectric thin films are being developed.
Piezoelectric devices include a composite piezoelectric substrate including a piezoelectric thin film and a supporting substrate (see, for example, Japanese Unexamined Patent Application Publication No. 2002-534886).
In accordance with Japanese Unexamined Patent Application Publication No. 2002-534886, the supporting substrate may be made of sapphire, silicon, or gallium arsenide, and the piezoelectric thin film may be made of a piezoelectric substance, such as quartz, lithium tantalate (LT), or lithium niobate (LN). Japanese Unexamined Patent Application Publication No. 2002-534886 also discloses a method for manufacturing a composite piezoelectric substrate in which a piezoelectric thin film is formed by dividing a piezoelectric substrate after ion implantation.
In this method, the piezoelectric substrate has a thickness sufficient for bonding, and ions, such as hydrogen ions, are implanted into one main surface of the piezoelectric substrate to form an ion-implanted layer. This main surface is then bonded to a supporting substrate by activated bonding or affinity bonding. The piezoelectric substrate is then heated to be divided at the ion-implanted layer to form the piezoelectric thin film.
Such a method can produce a composite piezoelectric substrate including a very thin piezoelectric film supported by a supporting substrate. However, implanted ions remaining in the piezoelectric thin film adversely affect the piezoelectricity of the composite piezoelectric substrate. Thus, in order to recover the piezoelectricity of the composite piezoelectric substrate, the piezoelectric thin film is sometimes heated at a temperature greater than the dividing temperature for a long period of time to remove the residual ions from the piezoelectric thin film.
Thus, in the method for manufacturing a composite piezoelectric substrate in which a piezoelectric thin film is formed by dividing a piezoelectric substrate at an ion-implanted layer, the piezoelectric thin film bonded to a supporting substrate is heated to a dividing temperature and an annealing temperature as described above. During these heating processes, a high thermal stress at the interface between the supporting substrate and the piezoelectric thin film can cause problems, such as the detachment or cracking of the piezoelectric thin film. Such defects in the piezoelectric thin film are particularly noticeable during the manufacture of large composite piezoelectric substrates, making it difficult to commercially manufacture large composite piezoelectric substrates. To avoid this, a supporting substrate must be made of a material that produces a low thermal stress during heat treatment. This imposes a significant constraint on the coefficient of linear expansion of the material.
Since the functions of piezoelectric devices depend on the characteristics of a supporting substrate, it is desirable to have various alternative suitable materials for the supporting substrate. In the case of devices for filter applications, a reduction in the coefficient of linear expansion of a supporting substrate can improve the temperature-frequency characteristics of a filter. However, due to the constraint on the coefficient of linear expansion, the material of a supporting substrate cannot have a coefficient of linear expansion much less than the coefficient of linear expansion of a piezoelectric thin film. It is desirable that the material of a supporting substrate have high thermal conductivity to improve the heat radiation characteristics and the electric power resistance of the supporting substrate. It is also desirable that the material of a supporting substrate be inexpensive in order to reduce the manufacturing costs of devices. However, such a material does not always satisfy the constraint on the coefficient of linear expansion. Furthermore, use of a material having high processibility, such as silicon, for a supporting substrate enables the supporting substrate to have a complicated structure. This permits a method for forming a piezoelectric thin film by dividing a piezoelectric substrate at an ion-implanted layer to be applied to various devices, such as micro-electro-mechanical systems (MEMS) and gyros. However, such a high-processibility material rarely satisfies the constraint on the coefficient of linear expansion. Thus, the material of a supporting substrate is significantly limited.
In accordance with Japanese Unexamined Patent Application Publication No. 2002-534886, electrodes are formed on the divided surface of the ion-implanted layer to produce a surface acoustic wave device. In this case, even after the piezoelectricity recovery treatment, ions remaining in the vicinity of the divided surface can cause significant piezoelectric degradation.